1. Field of the Invention
The present invention relates in general to a generally cylindrical, fluid-filled resilient bushing or mounting structure, adapted to exhibit excellent damping characteristic for low-frequency vibrations, and provide an effectively reduced dynamic spring constant for high-frequency vibrations.
2. Discussion of the Prior Art
A resilient mounting structure is known in the art of automotive vehicles, as a device for connecting members in a suspension system, or mounting a power unit on a body or frame of the vehicle. Such a mounting structure includes a resilient member interposed between radially spaced-apart inner and outer metal sleeves, and is installed such that the inner and outer sleeves are secured to suitable fittings for elastic connection of the two members of the vehicle, in such a manner as to primarily damp and isolate vibrations applied to the mounting structure in a diametric direction thereof, i.e., in a direction perpendicular to its axis. For example, such a resilient mounting structure is used as an engine mount bushing for mounting a power unit (including an engine) on a transverse F--F vehicle (front-engine, front-drive vehicle with its engine oriented transversely of the vehicle), such that the engine mount functions as a rolling stop for protecting the power unit from rolling vibrations.
In the recent years, various types of fluid-filled resilient bushings have been proposed as an improved resilient bushing, as disclosed in U.S. Pat. Nos. 3,642,268 and 3,698,703. These fluid-filled resilient bushings have vibration-isolating capability based on the elastic deformation of a resilient member, and vibration-damping capability based on a flow resistance of a fluid through the bushing and an inertia of the fluid mass, thereby exhibiting a comparatively low dynamic spring constant or rate for vibrations of a desired frequency range. More specifically described, the fluid-filled resilient bushings disclosed in the above-identified patents use a generally annular rubber block interposed between an inner sleeve and an outer sleeve, such that the rubber block partially defines a pair of fluid chambers which are filled with a suitable incompressible fluid. The bushings are adapted so that the fluid may flow through an orifice or orifices, from one of the two chambers to the other.
The fluid-filled resilient bushings mentioned above are required to demonstrate a sufficiently low dynamic spring contant for various frequency ranges of the vibrations to be applied thereto, depending upon the environments in which the bushings are used. For instance, an engine mount in the form of a bushing is required to provide a low dynamic spring constant for vibrations having comparatively high frequency ranges, to prevent booming noises and thru-engine noises.
In the fluid-filled resilient bushing or mounting structures, however, the cross sectional area and length of the orifice for fluid communication of the two fluid chambers are generally limited by the size and construction of the bushing structures. Usually, the orifice has a relatively large length and a relatively small cross sectional area. In this case, the bushing structures may provide a sufficiently low dynamic spring constant for low-frequency vibrations, for example, vibrations whose frequency is 50 Hz or lower. However, the dynamic spring constant of the bushing structures cannot be made low enough for vibrations having higher frequencies.